1. Field of the Invention
The present invention relates to an image processing apparatus, an image processing system, an image processing method, and a computer-readable storage medium which stores a program for causing a computer to realize functions of the apparatus and the system or to perform operations of the method. This image processing targets a radiograph of an object (imaged transmission amount of radiation having passed through the interior of the object) acquired by radiographing such as X-raying in the medical field.
2. Related Background Art
The interior of an object, particularly a human body has been observed by observing an X-ray amount transmission distribution via the object when the object is exposed to X-rays. In recent years, an X-ray distribution (image of which will be called an “X-ray image”) having passed through an object is generally acquired by a large-size image sensor using a solid-state image sensing element called a “flat panel X-ray sensor”.
One of the advantages of the solid-state image sensing element is to directly spatially sample a two-dimensional energy distribution (X-ray amount transmission distribution) by a plurality of receiving pixels present on a sensor surface, and convert the distribution into a signal.
As a disadvantage of the solid-state image sensing element, a plurality of pixel elements for spatially sampling an X-ray amount transmission distribution are basically independent elements having different characteristics. Acquiring an appropriate image (X-ray image) as if it was output from an ideal solid-state image sensing element made up of pixels having uniform characteristics requires image processing for correcting characteristic variations between pixels.
For example, when the pixel element is an energy conversion element having a linear characteristic, main characteristic variations are variations in conversion efficiency (gain) and offset. Hence, the gain and offset must be first corrected when the X-ray image of an object is to be obtained by an image sensor using a solid-state image sensing element.
As a method of correcting offset variations (to be also simply referred to as “offset correction” hereinafter), the unique offset value of each pixel attained without supplying X-ray energy to an image sensor is acquired. The offset value is subtracted from image information which contains an object image and is obtained by irradiating the image sensor with X-rays via the object. This method also allows setting an offset value acquisition time (e.g., energy accumulation time in the image sensor) in accordance with an actual X-ray image acquisition time.
As a method of correcting gain variations (to be also simply referred to as “gain correction” hereinafter), a so-called “white image” is acquired by supplying X-ray energy to an image sensor without any object. After the above-described offset variation correction is performed, gain correction is executed for each pixel by division using the white image. In most cases, gain correction is realized by the pixel value difference between pixels after logarithmic transformation.
The feature of an image sensor in acquiring an X-ray image is a very wide dynamic range of the X-ray amount received by the image sensor because of the following reason. To accurately draw information about the interior of an object, the X-ray amount irradiating the object is so adjusted as to make the dynamic range of an X-ray amount transmission distribution via the object coincide with the dynamic range of the image sensor. Then, an X-ray amount at a region (to be referred to as a “through region” hereinafter) where X-rays do not pass through the object increases and directly reaches the image sensor.
Direct incidence of too strong X-rays saturates an output value in an image sensor, i.e., an image sensor using a solid-state image sensing element. Such saturation occurs when the incident X-ray amount exceeds the charge allowable amount of a pixel on the X-ray image sensor, when an electrical amplifier on the output stage saturates, or when an A/D conversion system for digitizing an electric amount saturates. In any case, an obtained output value is fixed to an almost constant invariant value.
If offset correction or gain correction is done in this saturated state while correction data acquired in an unsaturated state is used as data used for each correction, variations between pixels to be corrected are conversely superposed on an image (e.g., through region), generating noise.
To solve this problem, e.g., Japanese Patent Application Laid-Open No. 2000-244824 proposes an arrangement in which a gain control amplifier adjusts a signal so as to always process its maximum value at any X-ray amount.
This arrangement, however, performs adjustment which accurately images even a direct ray region (through region on which X-rays are directly incident) having no information. This decreases the contract resolving power of a necessary object region.
Especially in an X-ray image, the dynamic range of image information is compressed to display and output the image information in order to make the details of the X-ray image clear. In this case, even a direct ray region containing many noise components is emphasized and output.
To solve this problem, e.g., Japanese Patent Application Laid-Open No. 6-292013 proposes an arrangement which decreases the emphasis degree of a direct ray region in compressing the dynamic range of image information. This proposal, however, does not specify a means for accurately extracting a direct ray region. Erroneous recognition of a pixel value which constitutes the direct ray region may degrade the contrast of an image diagnostic X-ray image.
Further, e.g., Japanese Patent Application Laid-Open No. 5-328357 proposes an arrangement which analyzes the peak value of a histogram obtained from an objective X-ray image to decide the dynamic range of the X-ray image. However, this arrangement may cause a large error in grasping an accurate pixel value of a direct ray region because only the peak value of the X-ray image histogram is analyzed.